Amish children raised on rural farms in northern Indiana suffer from asthma and allergies less often even than Swiss farm kids, a group known to be relatively free from allergies, according to a new study.

“The rates are very, very low,” said Dr. Mark Holbreich, the study’s lead author. “So there’s something that we feel is even more protective in the Amish” than in European farming communities. What it is about growing up on farms — and Amish farms in particular — that seems to prevent allergies remains unclear. Researchers have long observed the so-called “farm effect” — the low allergy and asthma rates found among kids raised on farms — in central Europe, but less is known about the influence of growing up on North American farms.

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Holbreich, an allergist in Indianapolis, has been treating Amish communities in Indiana for two decades, but he noticed very few Amish actually had any allergies. As studies on the farm effect in Europe began to emerge, Holbreich wondered if the same phenomenon might be found in the U.S. He teamed up with European colleagues to compare Swiss farming children and non-farming children to Amish kids in Indiana.

Amish families, who can trace their roots back to Switzerland, typically farm using methods from the 1800s and they don’t own cars or TVs. The researchers surveyed 157 Amish families, about 3,000 Swiss farming families, and close to 11,000 Swiss families who did not live on a farm — all with children between the ages of six and 12. They found that just 5% of Amish kids had been diagnosed with asthma, compared to 6.8% of Swiss farm kids and 11.2% of the other Swiss children.

Similarly, among 138 Amish kids given a skin-prick test to determine whether they were predisposed to having allergies, only 10 kids — or 7% — had a positive response.

In comparison, 25% of the farm-raised Swiss kids and 44% of the other Swiss children had a positive test, the researchers report in the Journal of Allergy and Clinical Immunology. The study did not determine why those who grew up on farms were less likely to develop asthma and allergies, but other research has pointed to exposure to microbes and cows. Drinking raw cow’s milk also seems to be involved, Holbreich said.

The theory is early exposure to diverse potential allergens and pathogens on a farm trains the immune system to recognize them, but not overreact to harmless ones. As for why Amish kids have even lower rates than other children, “that piece of the puzzle we really haven’t explained,” Holbreich said.

He speculated that it could be at least partly a result of the Amish having larger families or spending even more time outside or in barns than people on more modern working farms.

Dr. Corinna Bowser, an allergist in Cherry Hill, New Jersey, said there’s also a possibility that inherited factors could play a role.

“The Amish are still of a limited genetic pool, I would assume, because they’re much more segregated than the Swiss kids are,” she said.

Holbreich said upcoming studies will have an eye toward designing possible interventions. For instance, pregnant mothers could be exposed to factors that seem to protect farm kids as a preventive treatment, he explained.

“The goal is to try to find a way to prevent this allergy and asthma epidemic that western populations are facing,” Holbreich said.